The present invention relates to a device for producing artificial reverberation by means of a one-dimensioned waveguide. Such a device is known from Austrian Pat. Nos. 279,203 and 292,332 (U.S. Pat. Nos. 3,566,310 and 3,697,059).
Devices for the production of artifical reverberation resolve the unreverberated signal dispersively in a random manner. Such devices should, therefore, have a dense, irregular pole/zero distribution in the frequency spectrum to prevent flutter echo and tone shading. Preferably the frequency response of the reverberation time should decrease with increasing frequency.
In addition to the known devices for producing artificial reverberation, such as the reverberant chamber, the plate reverberator and the torsional vibration waveguide, it has become possible in recent times to construct electronic reverberation-producing devices using digital techniques. The advantages of digital devices for the production of artificial reverberation over the conventional electromechanical reverberators are their independence of environmental factors, the programmability of all reverberation parameters, their mechanical sturdiness, and the possibility of integrating them into the digital equipment of a sound studio.
For digital production of artificial reverberation, the use of rapid computers or digital signal processors is known, which simulate the transmission behavior of a room in real time. Such a signal processor consists of an analog/digital converter, which samples the unreverberated input signal and converts it into pulse code modulation, a high speed computer specifically designed for the reverberation which processes the coded sampled data, and a digital/analog conveter which re-converts the computer results into a continuous signal. Until now, reverberation equipment has become known in which high-quality resonators are simulated digitally. A loop composed of a digital delay arrangement, a digital damping element, and an adder is used as a resonator. The feedback is effected by multiplication of the feedback signal by a damping factor, which for reasons of stability must be less than 1. The pulse response is a periodical, exponentially decaying pulse sequence, and the frequency response meets the requirements of a periodical comb filter whose natural resonances occur as integral multiples. This periodicity is the main disadvantage of the resonators, and with long loop delays produces a flutter echo and, with short loop delays, a strong tone color. According to the state of the art, there are the following possibilities for weakening these two undesirable effects:
(1) Tapping the loop delay arrangement at several points, multiplying the signals occurring at the taps by different coefficients, and combining them additively as one output signal.
An equivalent circuit is a series-connected comb filter with irregular frequency response, which diminishes the tone color or timbre of the sound.
(2) Using several parallel- and/or series-connected, different resonators, the individual tone colors mask each other to a certain extent.
(3) Supplementing the arrangement described under point 2 by a feedback loop. This results in a reverberation system with multiple feedback and irregular natural resonance distribution. However, such a circuit structure can be realized only using analog filter techniques. In digital filter techniques, a disturbance referred to as a limit cycle occurs. Limit cycles are numerical instabilities which are caused by a buildup of the rounding error, which is inevitable in a multiplication. They are noticed as soft whistling sounds, which do not disappear even after the signal which triggered them has decayed.
Another disadvantage of resonator reverberation is that the reverberation time frequency response is flat and cannot be influenced. Digital resonators, in fact, have the property that all their natural resonances decay equally fast.